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Like his polar opposite, the Hollow Man, a lot remains unrevealed about the Mill Creek Killer, even after his arrest. The two eventually, more or less, began competing with each other in a form of rivalry, as they operated at the same time. They even communicated using personal ads under the aliases "Sunny" and "Holden", references to the novel The Catcher in the Rye.

The Mill Creek Killer is first seen approaching mother Ellen Carroll when she becomes secluded from her family in a bustling park. He catches her attention by stating he lost his daughter and stops her from calling her husband for help. He then pretends to hear his "daughter" move around in the nearby trees and lures Ellen behind a bush, where he punches her hard and drags her away as she dies. He is then seen tending to her corpse before engaging in sexual activities with it off-screen. It is Ellen's death, along with the Hollow Man's latest murder, that brings in the BAU. He is later seen approaching jogger Meredith Dale when she enters a secluded location, killing her and placing her inside his van. He encounters a security guard who inspects the van, but the guard fails to spot the body and he is allowed to go ahead. The Mill Creek Killer later dumps Meredith's body in the woods and it is discovered before he could engage in sexual activities with it. The BAU intends to leave the corpse there so they can wait for the Mill Creek Killer to return and engage in necrophilia with it so they can catch him in an act of irony, but when they apprehend a man who wanders toward the corpse, it is revealed he learned of the discovery and sent the man to locate it so the authorities would be tricked.

The Mill Creek Killer later attempts to lure a pedestrian into a secluded area, but the woman refuses, causing him to become volatile. She finally realizes that she is about to be abducted and manages to escape him and alert several people nearby, forcing him to flee in a rage. The BAU gets a physical description from the woman, and after they find out about how the two unsubs are communicating, they decide to set another trap for him: get a woman covered in leaves somewhere in the Mill Creek Killer's comfort zone, use the Hollow Man's alias to tell the Mill Creek Killer that he committed a copycat murder, and wait for him to approach the corpse. The ruse works and the Mill Creek Killer is arrested. He later pleads with Gideon to not reveal his necrophiliac activities to the public. When the Hollow Man, enraged at a speech JJ made about him being nonexistent and the murders being merely unconnected, arrives and holds a security guard hostage, he spots the Mill Creek Killer, realizes it is him, and accidentally puts down his gun, allowing the authorities to make an arrest.

Reid would later mention the Mill Creek Killer in a seminar on sexual sadism, which Nathan Harris attended.

The Mill Creek Killer targeted Caucasian women who were educated and lived in middle-class communities. He would kill them using blunt-force head trauma of some kind, abduct them, and hide their bodies in a ranger district of Mark Twain National Forest that wasn't being patrolled by the rangers, so he could revisit them later. These bodies were hidden under leaves and other debris. During the aforementioned revisits, he would apply makeup, groom their hair, and engage in sexual activities with the corpses.

The Mill Creek Killer is a sexually-motivated offender driven by internal forces and uses blunt-force trauma in order to feel the lives leave his victims' bodies. He spends a lot of time with his victims before and after he kills them. Because his victims willingly follow him in broad daylight, he appears harmless. He would be handsome and have the social skills required to lure his victims. Those who knew him well would be shocked when told that he was the killer. He has been able to get his victims away from their families and friends, making him feel powerful. His ritual of driving his victims from the abduction site to the woods has become the most important thing to him; it dominates his thoughts, which provide the privacy he needs. His sexual motivations make him more predictable, but that does not make it easier for police to catch him. Since he is well aware of the patrol schedules of the forest rangers working in the woods where he dumps his victims' bodies, it is possible that he works there himself.

The Mill Creek Killer has some resemblance to Ted Bundy, the two having a similar victim type (brunette young women) and M.O. (luring with a ruse, then bludgeoning). The two would also return to their dump sites in the woods to groom and have sex with the corpses of their victims until the decomposition became too severe.